Impact beam comprising precipitation hardenable stainless steel

ABSTRACT

The present invention relates to an impact beam for use in a vehicle. More specifically, the invention relates to an impact beam comprising precipitation hardenable stainless steel, and a method of producing such a beam. The precipitation hardenable stainless steel has a composition, all in percent by weight, of: C max 0.07 Si max 1.2 Mn max 0.7 Cr 10-14 Mo max 1.5 Ni 7-12 Cu max 2.6 Ti 0.6-2.0 (Nb+Ta) max 0.7 balance Fe and normally occurring impurities. An impact beam comprising precipitation hardenable stainless steel, according to the invention, provides improved impact absorbing properties per unit of weight, and can be formed by conventional hot forming techniques.

FIELD OF INVENTION

The present invention relates to an impact beam for use in a vehicle. More specifically, the invention relates to an impact beam and related assembly, the impact beam comprising precipitation hardenable stainless steel, and a method of producing such a beam.

BACKGROUND

Increasing application demands and strict regulations regarding accidental safety and environmental impact have resulted in a need for manufacturers of vehicles, such as automobiles, buses and motorcycles, to reduce both cost and weight of components for crash protection with maintained safety. Lower weight provides several advantages in an environmental context, for example, decreased fuel consumption resulting in reduced emission of harmful exhaust gases. Consequently, the development of new, high strength materials for use in vehicle components, such as impact beams, is of vital importance.

Impact beams can, for example, be designed to protect passengers in the vehicle by absorbing impact energy in a collision, through plastic and/or elastic deformation. Furthermore, impact beams can also be designed to protect objects outside the vehicle, such as pedestrians or animals. In a collision between two or more vehicles, for example in a head-on collision, an impact beam can limit the damage caused by the colliding vehicle to the oncoming vehicle, resulting in less risk of passengers in the oncoming vehicle being seriously injured. Impact beams can also be used to prevent vital machinery inside the vehicle from being damaged. In addition to absorbing impact energy an impact beam can also be designed to transmit impact energy to the vehicle frame, i.e. the chassis without the running gear, and/or the vehicle body structure, such as a door, or another impact beam. Running gear in this context includes, inter alia, engine, drive shaft, transmission and suspension.

High strength carbon steel (tensile strength<800 MPa) is commonly used for vehicle components designed for crash protection due to physical properties, such as high strength, good formability, and inherent capability to absorb impact energy in a crash situation.

Components manufactured from high strength carbon steel are heavy, which results in a heavy vehicle and thereby accompanying drawbacks, such as high fuel consumption. When high strength in combination with low weight is required ultra-high strength carbon steels (tensile strength>800 MPa), for example boron steels, can be used. However, ultra-high strength carbon steels may involve formability problems as well as low ductility, which can lead to brittle cracking.

An example of an impact beam is disclosed in EP 1520741, wherein the beam is described as an automobile strength member comprising a rectangular steel tube. Brittle cracking is identified as a problem, which arises for high strength members having tensile strengths exceeding 1470 MPa. The preferred production methods include e.g. drawing, rolling or extrusion.

In WO 02/064390, manufacturing of a lightweight vehicle door, comprising a supportive door frame that includes at least one impact beam, by hot forming particularly hot stamping, is disclosed. Tensile strengths of over 1000 MPa up to 1500 MPa are expected when using boron steel.

Conventional manufacturing processes of vehicle components generally use hot forming. To avoid additional costs connected to exchanging the machinery in the production units, it is a considerable advantage if the components can be manufactured by conventional hot forming techniques. Consequently, one object of the present invention is to provide an impact beam for use in vehicles, which provides improved impact absorbing properties per unit of weight, wherein the impact beam can be formed by conventional hot forming techniques.

SUMMARY OF THE INVENTION

The stated object is achieved by the present invention in accordance with claim 1. The impact beam, according to the invention, comprises precipitation hardenable stainless steel, wherein the steel has a composition, all in percent by weight, of:

C  max 0.07 Si max 1.2 Mn max 0.7 Cr 10-14 Mo max 1.5 Ni  7-12 Cu max 2.6 Ti 0.6-2.0 (Nb + Ta) max 0.7 balance Fe and normally occurring impurities.

Use of a precipitation hardenable stainless steel, according to the present invention, in an impact beam, provides a vehicle component with improved impact absorbing properties per unit of weight, which may for example facilitate significant weight reduction of the impact beam while preserving the same energy absorption properties.

The present invention also relates to a method of producing an impact beam, comprising said precipitation hardenable stainless steel by hot forming, such as for example hot stamping or press hardening. Press hardening is a manufacturing process for low weight, ultra-high strength components, in which simultaneous forming and quenching is utilized. By using press hardening formation of complex geometries is made possible due to the high formability of the hot steel, and the quenching results in a component with very high yield and tensile strength, as well as high dimensional accuracy. Furthermore, new design opportunities are available and complex designs are enabled, which, for example, may lead to space savings when assembling the vehicle body. The design can also aim at controlling the absorption of the impact energy, by controlling the deformation of the beam.

By using conventional manufacturing techniques, such as press hardening, additional costs connected to exchanging the machinery in the production units, can be avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates examples of impact beams in an automobile, which can comprise precipitation hardenable stainless steel, according to the present invention.

FIG. 2 illustrates a vehicle door with a waist rail reinforcement beam and a side impact beam.

FIG. 3 illustrates a cross section having two intersecting tangents.

FIG. 4 illustrates examples of cross section geometries for impact beams.

FIG. 5 illustrates examples of complex cross section geometries for impact beams.

FIG. 6 illustrates the setup used in the FEA (Finite Element Analysis) for a beam with circular cross section.

FIG. 7 illustrates the setup used in the FEA for a beam with C-shaped cross section.

FIG. 8 illustrates results from FEA comparing steel according to the present invention to boron steel of type Mat. No. 1.5528, using a beam with circular cross section and a wall thickness of 1.5 mm.

FIG. 9 illustrates results from FEA comparing steel according to the present invention to boron steel of type Mat. No. 1.5529, using a beam with circular cross section and a wall thickness of 1.5 mm.

The figures should not be considered drawn to scale, since some features may have been exaggerated in order to clearly illustrate the invention.

DETAILED DESCRIPTION OF THE INVENTION

To achieve a substantial weight reduction, while strength and energy absorption is sustained, the impact beam should comprise steel with ultra-high strength (>1000 MPa). Ultra-high strength precipitation hardenable stainless steel provides high tensile strength combined with excellent impact absorbing properties, and is an alternative to ultra-high strength carbon steel. When using steel with higher strength, the amount of material in the component can be reduced due to the improved energy absorbing capacity per unit of weight. Thereby, the total weight of the component, and in the end the weight of the vehicle, is reduced. This can be achieved by for example reducing the wall thickness of the impact beam.

Instead of obtaining reduced weight while impact absorbing properties remain unchanged, an increase of strength can be achieved by manufacturing an impact beam using an ultra-high strength steel, with preserved amount of impact absorbing material.

An impact beam designed for crash protection, as described in the present disclosure, can be used in several types of vehicles, such as automobiles, motorcycles, buses, trucks, caterpillars, crawlers, and tractors. The beam can be joined to, or be a part of, the vehicle frame, i.e. the chassis without the running gear, or the vehicle body structure. The impact beam can also be used in other types of vehicles such as motorboats, snowmobiles, or airborne vehicles such as helicopters or airplanes. For example, impact beams are important components in the floor structure of helicopters.

FIG. 1 illustrates examples of impact beams in an automobile. The following components are identified in the figure: bumper beam 1, side member 2, A-pillar reinforcement 3, front header 4, roof beam 5, B-pillar reinforcement 6, floor beam 7, door beam 8, cross member 9 and waist rail reinforcement 10. Impact beams, which are situated in more than one location for example side members 2 and A-pillar reinforcements 3, are normally placed at corresponding sides in the vehicle and are therefore not indicated in FIG. 1. Examples of impact beams in a vehicle door 21 comprising a waist rail 22 and a side impact beam 23 are illustrated in FIG. 2.

In this context a beam is considered as a structure comprising a cross section with at least two intersecting tangents, illustrated in FIGS. 3 as t1 and t2 for an angle beam 31, and t1′ and t2′ for beam with a circular cross section 32, wherein the tangents define a two-dimensional plane, and wherein the structure is extended in a direction essentially perpendicular to the plane. The beam can be designed in a number of different shapes and sizes. FIG. 4 shows some examples of basic cross sectional shapes of impact beams: circular 41, elliptical 42, U-shaped 43, C-shaped 44 or hat shaped 45. The cross section can also be of essentially square shape, essentially triangular shape, essentially tetragonal shape, essentially pentagonal shape, as well as of irregular shapes.

The cross section of the beam can contain one or more open sections, such as the open area A defined by t1 and t2 in FIG. 3, and/or one or more closed sections, such as the area B in FIG. 3. Moreover, the shape and/or the size of the cross section can either be identical or vary throughout the extension of the beam.

By using a specific design, such as a specific cross section, the impact beam can be adapted so as to absorb impact energy, through controlled deformation of the beam. The impact beam can also be adapted so as to transmit impact energy to other parts of the vehicle frame and/or the vehicle body structure, thus directing the impact energy away from the impact zone.

In FIG. 5 examples of complex geometrical forms of cross sections of impact beams are illustrated. The cross sections illustrated in FIG. 5 are examples of a floor beam 51, a waist rail in a vehicle door 52, a side impact beam in a vehicle door 53 and a roof bow 54.

To avoid additional costs, associated with investments to modify the existing manufacturing process, the precipitation hardenable stainless steel should be suitable for hot forming. However, not all precipitation hardenable stainless steels can be formed by hot forming techniques without becoming too hard during the process. If the hardness of the steel increases too much during the hot forming process, it can lead to detrimental brittle fractures and poor impact absorbing properties, which is not desirable in an impact beam.

Three examples of precipitation hardenable stainless steels that meet the requirements stated above, inter alia excellent impact absorbing properties and possibility of hot forming are UNS S45500, UNS S45503 and UNS S46500. According to a preferred embodiment the precipitation hardenable stainless steel is of UNS S45500 type. Compositions of said precipitation hardenable stainless steels, in percent by weight, are displayed in Table 1. The balance is Fe and normally occurring impurities.

TABLE 1 Element UNS S45500 UNS S45503 UNS S46500 C  max 0.050  max 0.010  max 0.020 Si max 0.50 max 1.00 max 0.25 Mn max 0.50 max 0.50 max 0.25 Cr 11.0-12.5 11.0-12.5 11.0-12.5 Mo max 0.50 max 0.50 0.75-1.25 Ni 7.50-9.50 7.50-9.50 10.75-11.25 Cu 1.50-2.50 1.25-1.75 0 Ti 0.80-1.40 1.00-1.35 1.50-1.80 Nb + Ta 0.01-0.05 0.10-0.50 max 0.01

The precipitation hardenable stainless steel, according to the invention can be processed in the shape of a tube, sheet or bar, for further forming into various geometrical shapes. Furthermore, the impact beam can either consist entirely of precipitation hardenable stainless steel, according to the invention, or comprise a member of a precipitation hardenable stainless steel in combination with another member of another material, for example other steel grades or carbon fiber.

In Table 2, materials currently used in impact beams are compared with the precipitation hardenable stainless steel used according to the invention, in terms of properties, which are important for the intended use of the impact beam. Commonly used stainless steel in this application is for example AISI 301 type, and commonly used boron steel is, for example, material number (Mat. No.) 1.5528 or Mat. No. 15529.

TABLE 2 Precipitation hardenable Stainless Boron stainless steel used Properties steel type steel type according to the invention Tensile <1300 MPa <1400 MPa <1700 MPa strength Ductility 5-10% 5-10% <5-10% Formability Medium Medium Medium Weldability/ Good Good Good Joinability Corrosion Good Poor Good resistance Energy Medium Medium Excellent absorption

As is commonly known the material strength is affected by the degree of processing and the conditions of any heat treatments performed.

In an embodiment of the invention the impact beam is manufactured by any conventional hot forming technique. The temperature applied during hot forming is generally equal to, or exceeding, 750° C., typically around 900° C. Preferably the hot forming technique is press hardening. Preferably, the press hardening can be followed by a precipitation hardening step. The starting material for press hardening is usually in the form of a sheet, a tube or a strip, preferably the starting material is a steel sheet. The impact beam may be shaped to fit an available space in the vehicle and/or shaped to provide the best impact absorption.

The impact beam can be a part of an impact beam assembly, wherein the impact beam is joined to at least a part of the vehicle frame, and/or the vehicle body structure, for example a vehicle door or another impact beam, by conventional techniques, for example, bolting, welding, gluing or seaming.

In an embodiment of the invention, at least a part of the surface of the beam is pre-treated to improve the shearing strength of an adhesive joining. For example, the surface can be ground and/or chemically treated to remove most of the native oxide scale and thereafter coated with a primer, such as a silicon based primer. The primer will create a surface structure which interacts well with the glue and thereby strengthens the glued joint. Surface pre-treatment using a primer is performed after hot forming.

Example 1

Energy absorption in an impact beam comprising precipitation hardenable stainless steel, according to the invention, was studied using finite element analysis (FEA). An impact beam comprising precipitation hardenable stainless steel of the type UNS S45500 was used in the calculations, and the chemical composition of the steel is displayed in Table 1. The results were compared to FE-analyses performed for beams comprising two different conventional boron steels: Mat. No. 1.5528 and Mat. No. 1.5529. Compositions of said boron steels, in percent by weight, are displayed in Table 3. The balance is Fe and normally occurring impurities.

TABLE 3 Element Mat. No.* 1.5528 Mat. No. 1.5529 C 0.19-0.25 0.25-0.30 Si ≦0.40 ≦0.40 Mn 1.10-1.40 1.10-1.30 P 0.025 0.025 S 0.015 0.025 Cr 0.15-0.35 ≦0.50 Al 0.020-0.060 ≧0.020 Ti 0.0020-0.0050 0.020-0.050 B 0.0008-0.0050 0.0008-0.0050 *Mat. No. = Material number, also known as Werkstoff number.

Calculations were performed for beams with two different shapes: circular 41 and U-shape 43. The cross sections were identical throughout extension of the beam. Table 4 displays the input dimensions of the beams. In FIG. 6 the setup used in the calculation for a beam 61 with a circular cross section is shown and in FIG. 7 the setup used for a beam 71 with a U-shaped cross section is shown. The force was applied perpendicular to the extension of the beams, using a solid body 62, 72 with circular cross section, wherein the beams were fixed at the ends. Table 5 displays the material input data, wherein the precipitation hardenable stainless steel and the boron steel are in precipitation hardened state and hardened state, respectively.

TABLE 4 Circular U-shape Length (mm) 1000 Length (mm) 1000 Diameter (mm) 100 Height (mm) 100 — — Width (mm) 100 Weight at wall 2.43 Weight at wall 2.44 thickness 1.0 mm (kg) thickness 1.0 mm (kg)

TABLE 5 Input Data UNS S45500 Mat. No. 1.5528 Mat. No. 1.5529 E (GPa) 200 200 200 R_(p0.2) (MPa) 1600 1150 1350 R_(m) (MPa) 1800 1550 1700 A80 (%) 10 9 6

For each cross sectional shape, calculations were performed for three different material thicknesses. When comparing a beam comprising precipitation hardenable stainless steel according to the invention, and a beam comprising boron steel of type Mat. No. 1.5528, material thicknesses of 0.8 mm, 1.0 mm and 1.5 mm were used, see Table 6. When comparing a beam comprising precipitation hardenable stainless steel according to the invention, and a beam comprising boron steel of type Mat. No. 1.5529, material thicknesses of 1.0 mm, 1.5 mm and 2.0 mm were used, see Table 7. All steel types used in the FEA have approximately the same density, which means that the result can be used to estimate the weight savings of the final component.

The results from the FE-analysis regarding beams with circular cross section and U-shaped cross section, comprising the precipitation hardenable stainless steel according to the invention and a beam comprising boron steel of the type Mat. No. 1.5528 are displayed in Table 6. Results regarding the comparison with boron steel of the type Mat. No. 1.5529 for the above mentioned cross sectional shapes are displayed in Table 7. FIG. 8 and FIG. 9 display results from the calculations using a beam with circular cross section, with a thickness of 1.5 mm, for the comparisons with Mat. No. 1.5528 type steel and with Mat. No. 1.5529 type steel, respectively.

TABLE 6 Circular U-shape Energy absorbed in Energy absorbed in structure (Nm, J) Differ- structure (Nm, J) Differ- Thickness UNS Mat. No. ence UNS Mat. No. ence (mm) S45500 1.5528 (%) S45500 1.5528 (%) 0.80 614 520 18% 896 812 10% 1.00 972 809 20% 1262 926 36% 1.50 2259 1810 25% 2582 2068 25%

TABLE 7 Circular U-shape Energy absorbed in Energy absorbed in structure (Nm, J) Differ- structure (Nm, J) Differ- Thickness UNS Mat. No. ence UNS Mat. No. ence (mm) S45500 1.5529 (%) S45500 1.5529 (%) 1.00 993 957 4% 993 938 6% 1.50 2315 2177 6% 2191 2016 9% 2.0 3939 3653 8% 3876 3512 10% 

The results show that the beam comprising the precipitation hardenable stainless steel according to the invention, displays an increased energy absorption, in beams with both circular and U-shaped cross sections, by on average at least 20% compared to a beam comprising boron steel of the type Mat. No. 1.5528. Compared to a beam comprising boron steel of the type Mat. No. 1.5529, the beam according to the invention displays an increased energy absorption by on average at least 7%. The true energy absorption may be even higher than suggested in these FE-analyses due to the large elongation to fracture in the beam according to the invention, compared to a beam comprising boron steel. Impact beams with lower fracture toughness can experience cracking when the beam is deformed, which locally leads to a considerable reduction of the ability to absorb energy.

Example 2

Energy absorption in an impact beam with C-shaped cross section 44, comprising precipitation hardenable stainless steel of UNS S44500 type and conventional boron steel of type Mat. No. 1.5529 was studied by FEA. The material input data and experimental setup were the same as described in Example 1. The input dimensions of the C-shaped beam were:

-   -   Length: 1000 mm     -   Height: 100 mm     -   Width: 100 mm     -   Weight at wall thickness 1.0 mm: 2.45 kg.

Results are displayed in Table 8.

TABLE 8 C-shape Energy absorbed in structure (Nm, J) Thickness UNS Mat. No. Difference (mm) S45500 1.5529 (%) 1.00 1017 924 10% 1.50 1731 1272 36% 2.0 3203 2780 15%

By using an impact beam comprising precipitation hardenable stainless steel, according to the present invention, which has up to 50% higher tensile strength than conventional steel types used for impact beams, a considerable weight reduction of at least 20% on average can be obtained for the final component. High tensile and yield strengths, of the precipitation hardenable stainless steel used according to the invention, in combination with high ductility and high toughness, result in a superior ability to absorb impact energy in a collision, through plastic and/or elastic deformation, making the steel highly suitable for use in impact beams. Also, the high elongation at rupture, associated with this precipitation hardenable stainless steel, results in less risk of cracking. Furthermore, since the precipitation hardenable stainless steel, used according to the invention, is corrosion resistant there is no need for any additional corrosion protection throughout the expected life time of the vehicle. 

1. Impact beam comprising precipitation hardenable stainless steel, wherein the steel has a composition, all in percent by weight, of: C  max 0.07 Si max 1.2 Mn max 0.7 Cr 10-14 Mo max 1.5 Ni  7-12 Cu max 2.6 Ti 0.6-2.0 (Nb + Ta) max 0.7 balance Fe and normally occurring impurities


2. Impact beam, according to claim 1, wherein the beam is adapted so as to absorb impact energy through deformation of the beam.
 3. Impact beam, according to claim 1, wherein the beam is adapted so as to transmit impact energy to an adjacent structure.
 4. Impact beam according to claim 1, wherein the stainless steel is UNS S45500.
 5. Impact beam according to claim 1, wherein the stainless steel is UNS S45503.
 6. Impact beam according to claim 1, wherein the stainless steel is UNS S46500.
 7. Impact beam according to claim 1, wherein the steel is precipitation hardened.
 8. Impact beam according to claim 1, wherein the beam structure comprises a cross section with at least two intersecting tangents defining a two-dimensional plane, wherein the structure is extended in a direction essentially perpendicular to the plane.
 9. Impact beam assembly, comprising an impact beam according to claim 1, and at least one part of a vehicle frame and/or a vehicle body structure.
 10. Impact beam assembly according to claim 9, wherein the impact beam is joined to at least one part of a vehicle frame and/or a vehicle body structure, by any technique.
 11. Method of producing an impact beam according to claim 1, wherein the impact beam is manufactured by hot forming.
 12. Method according to claim 11, wherein the impact beam is manufactured at a temperature equal to, or exceeding, 750° C., typically around 900° C.
 13. Method according to claim 11, wherein the hot forming technique is press hardening.
 14. Method according to claim 11, wherein the beam is subjected to precipitation hardening after hot forming
 15. Method according to claim 11, wherein at least a part of the surface of the beam is coated with a primer, such as a silicon based primer.
 16. Method according to claim 12, wherein the hot forming technique is press hardening 